1. Field of the Invention
The present invention relates generally to composite bodies or structures formed of structural fibers in a resin matrix. More specifically, the present invention relates to a braided fiber preform for use in composite molding processes that result in composite bodies or structures having improved strength characteristics.
2. Background Information
Composite materials consisting of fibers and a resin matrix are used to produce a wide range of useful products, from fiberglass sailboat hulls to the recent radar-transparent "Stealth" aircraft. Composite structures have a number of advantages, including strength-to-weight-ratios approaching or even surpassing those of the most advanced structural alloys.
Several processes or methods for forming composite bodies or structures are in conventional use. Generally, all of these methods involve the formation of a "layup" or preform of fibrous material, which generally takes the contours of the finished composite structure. This layup or preform may be formed of a fabric of structural fibers or individual fibers themselves, and may be "laid up" against a mandrel either manually or by a mechanized apparatus. One such method of forming a preform or composite bodies or structures is to braid a plurality of structural fibers about a mandrel. An example of this method is found in U.S. Pat. No. 4,519,290, May 28, 1985 to Inman et al., which discloses a braided preform fabrication for a refractory article such as an exit cone of a rocket motor nozzle.
One shortcoming of prior-art composite materials and structures, particularly those employing graphite fibers, is that the resulting composite structures have generally satisfactory tensile strength, but compressive strength that is only a fraction of the tensile strength. A recent improvement in composite structure technology is found in commonly assigned U.S. Pat. No. 5,324,563, Jun. 28, 1994 to Rogers et al., which discloses a pultruded rod of carbon fibers having an amplitude to length (A/L) ratio of less than 0.9% disposed in a matrix that is solidified or cured into a rigid form. The composite structure disclosed in this patent has a compressive strength that approaches its tensile strength and provides a vastly improved composite structure. However, due to the recency of this improvement, there are relatively few applications for this marked improvement in composite structure technology.
A need exists, therefore, for an improved preform for use in composite molding processes and the composite structures or bodies resulting therefrom that incorporates the recent advances in composite technology in which the compressive strength of composite bodies or structures approaches the tensile strength thereof.